This invention relates generally to radar systems of the moving-target-indicator type and, more particularly, to arrangements which compensate for the effects of platform motion or antenna scanning in such radar systems.
A coherent moving-target-indicator (MTI) radar system uses the doppler characteristic present in the backscattered radar pulse to distinguish between the returns from moving targets and those from stationary objects or clutter. In a stationary system, the spectral content of a received signal due to clutter is essentially the same as that of the transmitted pulse and only the spectral content of a received signal due to a moving target has a Doppler shift. However, in an airborne moving target indicator (AMTI) system, the clutter sources move relative to the radar platform so that the signals originating from them exhibit a Doppler shift. Consequently, both the clutter spectra and the moving target exhibit a Doppler shift in frequency as a function of platform velocity. In addition, the MTI detection problem is also complicated by the scanning motion of the radar antenna. The antenna rotation causes the received signals to experience an amplitude modulation because the signal gain changes as the antenna beam moves relative to the positions of the scatterers.
As is well known, platform-motion effects may be compensated by physically or electronically displacing the antenna's phase center along the plane of its aperture. A first pulse is transmitted and received with the antenna having its phase center at a location P1. A second pulse is transmitted and received with the antenna having its phase center at a location P2. With exact compensation, these phase centers P1 and P2 will be separated by an amount equal to the distance moved by the aircraft during this interpulse period, and so P1 and P2 coincide in space. Then, the signal received in the first channel from any stationary object energized by the first pulse will be identical to the signal received in the second channel on the second pulse. Commonly, it is desirable to leave the transmit antenna fixed with respect to the aircraft and separate the received antennas by twice the distance the aircraft moves in an interpulse period to obtain the same result to a close approximation. With this accomplished, two sets of return signals are available, almost identical with the pair which would be received if the platform were stationary. Techniques based on this principle are known as DPCA-Displaced Phase Center Antenna.
In one implementation of the DPCA technique, as described in Chapter 18 of "Radar Handbook," M. I. Skolnik (Editor), McGraw Hill, 1970, the signal returns are first formed into a sum channel and a difference channel. In first-order terms, if the difference pattern is in quadrature with the sum pattern and is proportional to the sum pattern multiplied by tan .eta., where 2.eta. is the pulse-to-pulse phase advance as seen by the radar receiver due to the platform motion, the difference channel may be used to compensate the sum channel for the effects of the platform motion. The two channels are combined in a hybrid amplifier which produces the sum and difference of the two channels. The clutter is then cancelled by subtracting the sum output of the amplifier, delayed by a time interval equal to the period between radiated pulses, from the difference output of the amplifier.
A similar technique, also described in "Radar Handbook", supra, is used to compensate the sum channel for the effects of the antenna rotation. A difference channel that is in phase with the sum channel and is proportional to the derivative of the sum channel is combined with the sum channel in a hybrid amplifier. The clutter may then be cancelled by subtracting the sum output of the amplifier, delayed by a time interval equal to the period between radiated pulses, from the difference output of the amplifier.
When implementation of such a system is attempted, a question immediately arises as to the choice of sum pattern to give optimal performance. Commonly, a low-sidelobe design has been adopted with the implied assumption that the motion compensation arrangements do not modify the sidelobe characteristics significantly. This is generally invalid, but because low-sidelobe levels have not been achieved in practice, this has not been important. With systems having truly low-sidelobe levels now of practical significance, it is appropriate to consider an optimal design for motion compensation. In addition, motion compensation in conventional cancellers has not been provided beyond the first stage of cancellation, and techniques of accomplishing this are now of practical interest.
Modern systems often call for higher-order processing such as N-pulse coherent integration where, for example, N can be equal to 16. The prior-art motion compensation, as incorporated in the conventional MTI canceller, restricts the system design because the pulses are compensated for motion two at a time and the effects of the motion reappear between pairs. When processing is to be carried out coherently over a large number of pulses, it is advantageous to compensate for motion over this same group of pulses. In principle, these new MTI techniques, such as coherent integration, do not depend on the use of a precanceller (i.e., clutter cancellation as the first stage of MTI processing). Consequently, in addition, techniques of compensating for motion which do not rely on the presence of a pre-canceller are of interest.